Electrostatic air filters of the charged media type are known in the art. Such filters are described in U.S. Pat. No. 4,549,887 and in the applicant's copending U.S. patent application Ser. No. 07/183,785. Electrostatic air filters of the charged media type may be generically described as having one or more dielectric fibrous filter pads sandwiched between opposing electrostatic charging elements and supported in an air handling system by a support frame. The fibres in the dielectric filter pads are polarized by an electrostatic field generated between opposing charging elements, one element being connected to a ground and the other element being connected to a high voltage, low amperage current supply. The electrical polarization of the filter fibres significantly enhances the removal of particulates from contaminated air. Fine dust, molds and other particles small enough to pass readily through uncharged fibre air filters, are successfully removed by electrostatic air filters of this type. Electrostatically charged media air filters remove air pollutants having diameters as small as 0.3 microns. Although electrostatic air filters of the charged media type represent a significant advance in the art of air filtration, these filters are not very efficient in removing odours or gases from filtered air. Odours and gases are molecular in structure and therefore too small to be efficiently removed from air filtered by prior art electrostatic air filters.
Carbon containing open celled foam sheet structures used for removing odours and gases in air handling and filtration systems, are widely available and known in the art. There are a variety of such filter constructions commonly available, being generally constructed from powdered carbon or activated charcoal in combination with a binding agent, frequently a plastic foam. Other constructions of odour and gas absorbing air filter structures are also commercially available. Any such structure is suitable for use as an electrostatic charging element for the purposes of the invention if it has the property of absorbing odours and gases from filtered air and is a conductor of electrical energy.
As stated above, electrostatic air filters of the charged media type include at least one dielectric fibrous filter pad sandwiched between a pair of opposing charging elements. Traditionally, metallic screen or mesh is employed as the charging elements in air filters of the charged media type. Although a metallic screen or mesh works efficiently as a charging element for dielectric fibrous filter pads, it does not contribute to the filtering capacity of an air filter. In accordance with the invention, a charging element is provided which not only serves to charge the dielectric filter pads of charged media type filters, but additionally absorbs odours and gases from the filtered air.
It has been further established that the present invention provides several unexpected advantages when employed as a charging element in an electrostatic air filter. Not only are odours and gases removed from the filtered air, the efficiency of an electrostatic air filter in accordance with the present invention is significantly enhanced in that more filtrates are removed from air than were removed by the prior art electrostatic filters described above. The carbon filled charging element also absorbs moisture from filtered air and, in extremely humid conditions, helps to keep the dielectric filter pads in a dry condition which enhances filter performance by inhibiting the leakage of electrostatic energy across the fibrous filter pads. Damp filter pads decrease the efficiency of the filter because of strength lost in the electrostatic field due to electrical arcing across the pads which is enhanced by the presence of moisture.
A further advantage of a charging element in accordance with the invention is the fact that odours are absorbed by the charging element instead of being trapped among the fibers of a filter pad. The prior art electrostatic filters referred to above partially remove certain odours, such as cigarette smoke, from filtered air. However, those odours tend to escape from the filter whenever the electrostatic charge of the filter is interrupted. Thus, a known filter which is exposed to the smoke of several cigarettes exudes an unpleasant odour whenever the electrostatic charge on the filter is lost. A filter in accordance with the invention, however, does not exude an unpleasant odour on loss of the electrostatic charge on the filter. This is a particular advantage for desktop air filters or filters installed in confined spaces such as elevators or automobiles.
It is an object of the present invention to provide an electrostatic air filter which removes odours and gases as well as particulates from filtered air.